This invention relates to a miniature electric motor. More particularly, the present invention is concerned with a miniature electric motor comprising a plurality of pole teeth of soft magnetic material, a magnet for producing torque between the magnet and the pole teeth, a coil for energizing the pole teeth and a yoke portion adapted to be energized by the coil and wherein the pole teeth adapted to be energized by the coil accommodated in a bobbin are fixed together by means of a non-magnetic material into a mass to form a composite pole teeth structure, thereby enhancing precision in assembling the constituent parts of the miniature electric motor, which leads to improved performance of the miniature electric motor.
Conventionally, a miniature electric motor is of a structure that pole teeth, a yoke , a casing, etc., each made of soft magnetic material, are assembled into a unit part construction and then a predetermined number of such unit part constructions are stacked one upon another. Therefore, cumulative error is produced in the mutual positional relationships between the unit part constructions stacked one upon another. Particularly, the pole teeth which are required to have a very high precision with respect to the mutual positional relationships are influenced to a great extent by the errors caused by the yoke and casing and the like, leading to fatal drawbacks, such as lowering of performance, especially, lowering of efficiency, of the miniature electric motor as the final product.
With respect to the foregoing drawback inevitably accompanying the conventional miniature electric motors, an explanation will be given referring to FIGS. 1 to 2 annexed hereto. FIGS. 1 to 2, respectively, are a sectional elevation of a representative form of conventional miniature electric motor and an exploded view of the main parts thereof. The exploded view is shown with an upper yoke, an exciting coil and a lower yoke cut in half.
In FIGS. 1 to 2, numeral 1 designates an upper yoke which also serves as a casing, and which have been produced by subjecting to press-processing a plate of a soft magnetic material such as pure iron or low carbon steel. The upper yoke 1 comprises a cylindrical peripheral wall portion 1a, an annular plate portion 1b and pole teeth 8 which are interconnected and formed integrally. Numeral 3 designates a lower yoke which has the same structure as that of the upper yoke 1, except that the former is smaller than the latter in outer diameter by the thickness of the latter. In each of the upper yoke 1 and lower yoke 3, the annular plate portion 1b, 3b has at its outer edge portion a cylindrical peripheral wall 1a, 3a extending perpendicularly to said plate portion, and has at its inner edge portion a plurality of pole teeth formed so as to extend in parallel with the above-mentioned cylindrical peripheral wall. The upper yoke 1 and lower yoke 3 are, as depicted, coaxially stacked through the exciting coil. The diameter of the inner peripheral portion of the annular plate portion 1b of 1 is the same as that of the annular plate portion 3b of 3. In each of the upper yoke 1 and lower yoke 3, pole teeth and spacings are alternately arranged, the width of each spacing being designed so as to be slightly larger than that of each pole tooth, thereby to form a family of pole teeth 8, 8'. With respect to 1 and 3, the number of pole teeth, dimension thereof, shape thereof and the like are the same. The outer diameter of the outer peripheral portion of the annular plate portion 3b of 3, namely, the outer diameter of the cylindrical peripheral wall 3a formed at the above-mentioned outer peripheral portion is slightly smaller than the inner diameter of the cylindrical peripheral wall 1a of 1. So, the outer periphery of the cylindrical peripheral wall 3a of 3 is closely fitted to the inner periphery of the cylindrical peripheral wall 1a of 1. With respect to the families of pole teeth 8, 8' of the mutually fitted 1 and 3, the respective pole teeth are inserted in the respective spacings in mutual relationship. In this instance, the respective pole teeth of a first family of pole teeth and the respective pole teeth, to be inserted in the respective spacings of the first family, of a second family of pole teeth opposite to the first family, should be alternately arranged with exactly equal intervals. Actually, however, it is extremely difficult or impossible to make such an arrangement, leading to lowering of performance of an electric motor as a final product.
In any way, as described above, the respective families of pole teeth of the upper yoke 1 and lower yoke 3 do, in combination, cooperate with the exciting coil 2 to form a stator of one phase. When the exciting coil is energized by an alternating current, in the families of pole teeth 8 and 8' respectively of 1 and 3, N and S poles are alternately developed according to time. With one combination producing one phase, the direction of rotation is not determined. For this reason, another combination of an upper yoke 5, an exciting coil 6 and a lower yoke 7 respectively having quite the same structures as those of 1,2 and 3 is provided in a lower decker to form a second phase. Thus, a directional rotation force is produced by a two-phase AC operation. Illustratively stated, using two pairs of families of pole teeth, each pair having two families of pole teeth mutualy inserted in the spacings of the families, four families of pole teeth in total are stacked. The upper stator comprising 1, 2, and 3 which constitute one phase of upper decker is angularly offset by an angle of 90.degree. in terms of an electric angle with respect to a lower stator comprising 5, 6 and 7 which constitute another phase of lower decker. The exciting coil 2 is wound around a bobbin, and, as shown in FIG. 1, is accommodated in a spacing defined by 1 and 3, particularly by the annular plate portions 1b and 3b , cylindrical peripheral walls 1a and 3a, and the families of pole teeth 8 and 8'. When an alternating current is applied to the exciting coils 2 and 6 at their terminals (not shown), a magnetic circuit is formed within the annular plate portions 1b and 3b and cylindrical peripheral walls 1a and 3a, thereby energizing the pole teeth 8 and 8'. To the exciting coils 2 and 6 are respectively applied electric currents with a phase difference of 90.degree. therebetween. A rotor 4 comprises a shaft and a two-decker magnetized cylindrical magnet coaxially surrounding the shaft and fixed thereto. The magnet has multiple poles, and N and S poles are alternately arranged circumferentially on the magnet. The number of poles of the manget is the same as that of the pole teeth opposite to the poles of magnet. A motor is assembled so that the rotor 4 is situated, through a spacing, in opposite relationship to the four families of pole teeth composed of two pairs of families of pole teeth, each pair having two families of pole teeth mutually inserted in the spacings of the families. An electric current is applied to the motor, there is produced torque. The rotor is rotatably held by a bearing 10 supported by a bearing support 9.
Each of the yokes 1, 3, 5 and 7 are produced by means of a press mold in an integral form. In that instance, the influence of the rolling orientation due to rolling of the material, influence of the clearance of a press mold, influence of the spring-back due to a press processing and the like are piled one upon another, so that the center of the circular arangement of pole teeth 8, 8' actually comes to be not in coincidence with the centers of the annular plate portions 1b, 3b and cylindrical peripheral walls 1a and 3a. Further, due to the three-dimensional press processing, the right angularity of the respective pole teeth relative to each annular plate portion 1b, 3b varies position to position, causing the center intended by designing to be different from the center obtained in the actual product. Furthermore, the center of the annular plate portion 1b, 3b is not in coincidence with the center of the cylindrical peripheral wall due to crooking of the cylindrical peripheral wall. Therefore, when the yokes are assembled, the respective centers of the families of pole teeth stacked are not in coincidence with one another. The defects as mentioned above are attributed to the press processing and to the non-uniformity in the physical, mechanical properties of the metallic material due to rolling orientation, and cannot be actually avoided in the conventional technigues. In fact, when parts are assembled into a miniature electric motor, a combination of the yokes 1 and 3 mutually fitted each other and another combination of the yokes 5 and 7 mutually fitted each other are piled to form a four-decker construction. Therefore, due to the cumulative error produced by accumulation of the respective errors of the yokes, not only the distances of the respective pole teeth from the axis but also the spacings between every adjacent pole teeth become non-uniform and, hence, the scales of the spacings between the respective pole teeth and the rotor disposed coaxially with the circular arrangement of the pole teeth 8, 8' become different tooth by tooth, thereby causing it impossible to ensure high precision spacings between the rotor and the respective pole teeth. The measure that has been taken to solve the above-mentioned problem, however, is limited from the viewpoint of reduction in cost because of mass production which is carried out with respect to miniature electric motors, and is only to provide gaps with a large surplus and to escape from the difficulty caused by the above-mentioned cumulative error by utilizing the clearances produced by the large surplus gaps, resulting in no radical solution of the problem. With such a measure of increasing the clearances, a large magnetic loss is caused and desired amount of torque cannot be obtained, leading to lowering in performance of the miniature electric motor.
In an electric motor, the provision of the spacing between a stator and a rotor is the most important factor with respect to performance of the electric motor. It is preferred that the spacing between a stator and a rotor be as small as possible. For this purpose, it is necessary to enhance precision of the constituent parts. However, the conventional techniques suitable for improving precision of the constituent parts of a miniature electric motors are limited, and the development of new techniques useful for overcoming the difficulties accompanying the conventional miniature electric motors has been demanded in the art.
With a view to overcoming the above-mentioned drawbacks and to developing constituent parts of a miniature electric motor which are simple in structure and extremely effective for enhancing precision in assembling a miniature electric motor, the present inventor has made intensive investigations. As a result, it has been found that when there is used a unit in which pole teeth are fixed together by means of a non-magnetic material so that the pole teeth is unified with the non-magnetic material, with at least one of both surfaces of the pole teeth exposed, the spacing between a stator and a rotor in a miniature electric motor can be not only reduced by a factor of several as compared to that in the conventional miniature electric motor, but also the extremely uniform spacing can be ensured. The present invention has been made based on such novel finding.